The steps involved in the manufacture of an integrated circuit include the slicing of a semiconductor crystal into thin wafers, polishing of the wafers, preparation of masks or reticles, oxidation of the wafers, doping of the semiconductor material comprising the wafers, and processing the wafers through circuit forming steps such as photolithography, etching, exposure, etc. Once the circuits have been formed on a wafer, the individual circuits are probed to confirm that they are functional, and the wafer is then cut into chips and the individual chips packaged. At any one of these steps in the manufacturing process, it may be required to visually inspect the wafers for imperfections. In the past, the manufacturing steps, including the inspections, have typically been carried out in a clean room from which dust and other airborne particles have been substantially excluded.
Recently a system has been designed to permit the handling of semiconductor wafers both inside and outside of clean rooms. The system incorporates a standardized mechanical interface, and is commonly referred to by the acronym SMIF. The SMIF system includes dust-proof boxes for the transport and storage of the wafers. Referring to FIG. 1, such a box 10 is shown comprising cover 12, box door 14, and cassette or carrier 16 within the box and supported by the box door. Cover 12 is secured to box door 14 by a latch system indicated generally by reference numerals 18. Carrier 16 comprises a plurality of shelves arranged vertically one above the other, each shelf being adapted to hold one wafer or wafer-like object.
In the SMIF system, an arbitrary processing device 20 has a carrier port 22 through which carriers can enter and leave the device. Canopy 24 is connected to device 20 overlying the carrier port, and provides an enclosed, clean environment surrounding the carrier port. Incorporated into canopy 24 is port door 26 that is constructed to support and preferably mate with box door 14. Referring to FIG. 2, box 10 can be placed on port door 26, and the latch system is then actuated, the latch system being operative to release box door 14 from cover 12, and to connect the cover to the canopy. Port door 26 is then released from canopy 24, and the port door, box door and carrier may then be lowered into the space enclosed by canopy 24 by a suitable elevator 30. Once the carrier is within the canopy, it can be manipulated by arm 28 to move the carrier to carrier port 22, or for other operations.
The movement of the wafers to and from the carrier must be done with precision. Misalignment of the wafer as it is being placed on the carrier shelves can cause damage or breakage of the wafer. In addition, a wafer that is not fully seated within the carrier can impact on the fixed structure of the processing device as the carrier is unloaded. Furthermore, it is important for security and inventory purposes to account for the wafers as they are initially loaded into the processing device and when they are finally unloaded. In stations that utilize a second carrier for placement of rejected wafers, it is also important to know which shelves, if any, are occupied when the carrier is initially loaded into the processing device in order to avoid a collision that could result by attempting to place a wafer on an occupied shelf.